1. Field of the Invention
The present invention relates to a surface acoustic wave device to be used for a surface acoustic wave resonator or a surface acoustic wave filter and a method for manufacturing the same, and more specifically, a surface acoustic wave device having a bonding electrode in which a bonding wire is bonded via a wire bonding method, and a method for manufacturing the same.
2. Description of the Related Art
In manufacturing a surface acoustic wave device, a structure to seal the surface acoustic wave element in a package is often used. (For example, refer to Japanese Unexamined Patent Publication Nos. 50-68058, 51-17666, 61-172362).
The following method has been used in assembling a surface acoustic wave device to store the surface acoustic wave element in the package. First, the surface acoustic wave element in which at least one interdigital transducer (hereinafter, referred to as IDT) and a bonding electrode to be electrically connected to the outside are provided on a surface acoustic wave substrate is prepared. This surface acoustic wave element is fixed on an element mounting surface of a package member using an adhesive so as to constitute the package. Then, a terminal electrode to be electrically connected to the outside of the package member is bonded to the bonding electrode of the surface acoustic wave element via a wire bonding method making use of ultrasonic oscillation. That is, the bonding wire is bonded to the bonding electrode while applying the ultrasonic oscillation, and the other end of the bonding wire is bonded to the terminal electrode provided on the package member by the wire bonding method similarly applying ultrasonic wave oscillation.
After the connection by the above-described bonding wire is completed, another package member arranged to seal the surface acoustic wave element is fixed to the package member to complete the surface acoustic wave device.
In constructing the surface acoustic wave device to store the surface acoustic wave element in the package, the surface acoustic wave element is fixed to the element mounting surface of the package member using adhesive. However, the abovedescribed adhesive is shrunk during the curing process, and stress is exerted in the surface acoustic wave element due to this shrinkage during curing. Thus, there has been a problem that the characteristics of the surface acoustic wave device vary due to the shrinkage during curing.
In particular, the stress due to the above-described cure shrinkage is also varied by the operation environment or atmospheric change, and thus, the surface acoustic wave device characteristics vary during usage and operation.
On the other hand, when a strain is generated in the package to which the surface acoustic wave element is fixed, the stress is exerted in the surface acoustic wave element through the above-described adhesive. When the above-described stress is exerted, the characteristic of the surface acoustic wave element is varied. That is, there has been a problem that the variance in the stress causes the variance in the characteristics of the surface acoustic wave device, and the yield of the surface acoustic wave device is consequently degraded.
To overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave device and method of manufacturing the same, which minimizes the influence of stress exerted on the surface acoustic wave element by shrinkage during curing of the adhesive used to fix the surface acoustic wave element and the strain generated in the package, to thereby eliminate variations in device characteristics, and to achieve excellent bonding reliability of the bonding wire to the bonding electrode.
According to one preferred embodiment of the present invention, a surface acoustic wave device includes a package having an element mounting surface, a surface acoustic wave element provided in the package and having at least one bonding electrode, an adhesive arranged such that the surface acoustic wave element is fixed to the element mounting surface of the package, and at least one bonding wire connected to the at least one bonding electrode, in which the at least one bonding electrode has at least one line-shaped through hole extending therethrough at a region where the at least one bonding wire is connected to the at least one bonding electrode.
According to the surface acoustic wave device according this preferred embodiment, the surface acoustic wave element is fixed to the element mounting surface of the package through the soft adhesive layer, and the stress attributable to the cure shrinkage of the adhesive, the strain applied to the package, and other forces, are prevented from propagating to the surface acoustic wave element. Thus, the variations in the component characteristics are minimized, and the yield of the surface acoustic wave device are greatly improved.
The adhesive preferably has a Shore hardness of about 80 HSD or less in a cured state and may be one of modified epoxy resin adhesive and a silicone resin adhesive.
In addition, because the line-shaped through holes are formed in the above-described bonding electrodes, the oxide film on the surface of the bonding electrode is first removed by the sliding of the bonding wire at a peripheral edge portion of the through hole to start the bonding, and the bonded surface is extended in the direction of the ultrasonic oscillation when the bonding wire is bonded to the bonding electrode via the ultrasonic wire bonding method. Thus, the bonding strength and the bonding reliability of the bonding wire to the bonding electrode are greatly increased.
In addition, the through hole is line-shaped, the through hole can be easily formed through etching after the electrically conductive film is formed. Thus, the through hole can be easily formed without increasing the area of the through hole, and the bonding area of the bonding electrode is prevented from being reduced by forming the through hole. Because the area contributing to the bonding of the bonding electrode to the bonding wire is prevented from being reduced, the bonding strength and the bonding reliability of the bonding wire are also increased.
The line-shaped through hole may have a shape in which a plurality of line-shaped parts are combined. For example, the line-shaped through hole may have a zigzag shape or a cruciform shape.
In a case where a plurality of line-shaped parts are combined in the abovedescribed line-shaped through holes, the number of the edge portions from which the oxide film is first removed in the sliding direction of the bonding wire is increased compared with the case of the straight through hole, and thus, the bonding reliability of the bonding wire to the bonding electrode is further increased.
The method for manufacturing a surface acoustic wave device according to another preferred embodiment of the present invention preferably includes the steps of preparing a surface acoustic wave substrate, forming an electrically conductive film on the surface acoustic wave substrate, etching the electrically conductive film so that at least one interdigital transducer and a bonding electrode having a line-shaped through hole are formed, fixing the surface acoustic wave element to an element mounting surface of a package using an adhesive; and bonding a bonding wire to a bonding electrode of the surface acoustic wave element via ultrasonic wire bonding.
In the manufacturing method of a surface acoustic wave device of this preferred embodiment, at least one IDT and the bonding electrode having the line-shaped through hole are etched after the electrically conductive film is formed on the surface acoustic wave substrate. Thus, in the IDT forming process, the above-described line-shaped through hole can be simultaneously formed, and the surface acoustic wave device of the present invention can be easily provided without adding any extra process steps. Thus, the surface acoustic wave device of preferred embodiments of the present invention is achieved without increasing the time, expense and difficulty of the manufacturing process. A
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.